Methods for making chlorous acid and chlorine dioxide

ABSTRACT

Chlorous acid is generated from a chlorite salt precursor, a chlorate salt precursor, or a combination of both by ion exchange. The ion exchange material facilitates the generation of chlorous acid by simultaneously removing unwanted cations from solution and adding hydrogen ion to solution. Chlorine dioxide is generated in a controlled manner from chlorous acid by catalysis. Chlorine dioxide can be generated either subsequent to the generation of chlorous acid or simultaneously with the generation of chlorous acid. For catalysis of chlorous acid to chlorine dioxide, the chlorous acid may be generated by ion exchange or in a conventional manner. Ion exchange materials are also used to purify the chlorous acid and chlorine dioxide solutions, without causing degradation of said solutions, to exchange undesirable ions in the chlorous acid and chlorine dioxide solutions with desirable ions, such as stabilizing ions, and to adjust the pH of chlorous acid and Chlorine dioxide solutions.

FIELD OF INVENTION

The present invention relates to a method for generating chlorous acidfrom an aqueous chlorite salt solution or an aqueous chlorate saltsolution, or a combination of both solutions. The present invention alsorelates to a method for generating chlorine dioxide by means ofcatalysis of chlorous acid, either subsequent to or simultaneously withgeneration of the chlorous acid from a chlorite/chlorate salt solution.

BACKGROUND OF THE INVENTION

The generation of chlorous acid by the acidification of an aqueouschlorite salt solution or stabilized aqueous chlorine dioxide solution(stabilized chlorite salt solution) by an acid is well known by thefollowing reaction:

Na⁺ClO₂ ⁻+H⁺→H⁺ClO₂ ⁻+Na⁺  (1)

It is also well known that over time, chlorous acid slowly decomposes tochlorine dioxide by the following reaction:

5HClO₂→4ClO₂+HCl+2H₂O  (2)

This reaction (2) predominates at low acid and high chloriteconcentrations, making the reaction difficult to control, especially inhigh alkalinity water supplies. Further, this decomposition is slow. Atchlorite concentrations greater than 20,000 mg/L, the reaction rate is 5minutes at a pH<0.5. However, if the pH of the same chlorite solution isincreased to >1.0, the reaction is not complete after 60 minutes.

It is also well known that in an oxidizing environment, such as in thepresence of chlorine or an anode, chlorine dioxide can be generated fromchlorous acid by the following reaction:

HClO₂ −e ⁻→ClO₂+H⁺  (3)

It is further known that chlorous acid is generated by the acidificationof chlorate salt by the following two-step reaction:

Na⁺ClO₃ ⁻+H⁺→H⁺ClO₃ ⁻+Na⁺  (4)

HClO₃+HCl-->HClO₂+HOCl  (5)

In this reaction, hypochlorous acid, the ionized form of chlorine inwater, is also generated reaction (5). The generation of chlorinedioxide occurs via the one of the following mechanisms:

HClO₃+HClO₂-->2ClO₂H₂O  (6)

+

HOCl+HCl-->Cl₂+H₂O  (7)

or

2HClO₂+HOCl+HCl→2ClO₂+2HCl+H₂O  (8)

The generation of chlorine dioxide from chlorate salt, however, is verydifficult to control. In practice, excess acidity is required to startthe reaction, but if the acidity is too high, the following sidereaction predominates, and little to no chlorine dioxide is generated.

ClO₃ ⁻+6H⁺+6e ⁻-->Cl⁻+3H₂O  (9)

In practice, the following reduction reactions are used to generatechlorine dioxide from chlorate salt. High concentrations of allprecursors must be used to start the reactions, but when the reactionsdo not go to completion, undesirable byproducts or unreacted precursormaterials contaminate the chlorine dioxide solutions. In addition, thechloride ion must be present, either from the decomposition of chlorateor the addition of the chloride ion itself, for chlorine dioxide to begenerated. Overall reactions for the generation of chlorine dioxide fromchlorate are listed below.

2ClO₃ ⁻+SO₂-->2ClO₂+SO₄ ²⁻  (10)

4ClO₃ ⁻+CH₃OH+4H⁺-->4ClO₂+HCOOH+3H₂O  (11)

ClO₃ ⁻+Cl⁻+2H⁺-->ClO₂+Cl₂+H₂O  (12)

2ClO₃ ⁻+H₂O₂+2H⁺-->2ClO₂+O₂+2H₂O  (13)

It is further known that a mixture of chlorite salt and chlorate salt inthe presence of hydrogen ion will generate chlorine dioxide by thefollowing overall reaction:

2H⁺+ClO₂ ⁻+ClO₃ ⁻→2ClO₂+H₂O  (14)

This reaction is also shown in a different format in reaction (6).However, the rate of reaction to chlorine dioxide of the chlorite saltis 100 times faster than the rate of reaction to chlorine dioxide of thechlorate salt.

The use of chlorine dioxide in many applications has been limited due tothe inability to control the reaction chemistries and the inefficiencyof the reactions in solutions. Since chlorine dioxide is an unstablegas, even in solution, it must be generated on-site and used shortlyafter generation. Large-volume industrial applications such as pulp andpaper bleaching, municipal water pretreatment and disinfection, foodprocessing disinfection, and cooling tower disinfection have beensuccessful due to the ability in these applications to safely handleconcentrated and aggressive oxidizers and acids.

Some consumer and medical applications have also had success. Chloritesalt-based toothpastes, mouthwashes, and disinfecting skin gels useeither the pH of the mouth or a weak-acid activator to slightly acidifythe chlorite salt so that some chlorous acid is formed. The chlorousacid will then slowly decompose to chlorine dioxide by reaction (2).

It is also described in U.S. Pat. No. 6,200,557 BI that in a topicalsolution designed to treat HIV, the chemical addition of phosphates willretard the loss of chlorine dioxide from solution at pH 6-7.4. This isespecially beneficial in this topical application so that contact timeof chlorine dioxide on the skin surface is increased to allow bettertreatment of the HIV.

However, in all of the prior art processes, controlling the reactionshas remained a major obstacle. In addition, unreacted precursorcomponents and reaction by-products are undesirably carried over intothe product solutions. Also, in many instances, the pH of the product isso low due to the excess acid in solution that it cannot be used incertain applications.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been discovered thatchlorous acid can be generated in a controlled manner from an aqueouschlorite salt solution or an aqueous chlorate salt solution, or acombination of both, by ion exchange. It has further been discoveredthat chlorine dioxide can be generated from chlorous acid by the use ofat least one catalytic material. The chlorous acid for conversion tochlorine dioxide can be by ion exchange reaction in accordance with thepresent invention, or by conventional acidification. Preferred catalystsin accordance with the present invention include platinum, palladium,manganese dioxide, carbon and ion exchange material.

The chlorous acid may be generated separately in a first step andsubsequently catalyzed to form the chlorine dioxide in a second step, orthe chlorous acid and the chlorine dioxide may be generatedsimultaneously in the same reaction environment in the presence of therequisite catalyst. The process may be performed in either a continuousor a batch manner, and the reaction must be carried out in an aqueoussolution or otherwise aqueous moist environment, i.e., in the presenceof water or water vapor.

In the preferred embodiment of the present invention, the chlorous acidis generated by a salt cation/hydrogen ion exchange of chlorite salt orchlorate salt, or a combination of both, and the chlorous acid is thencatalyzed in a moist environment to form chlorine dioxide eithersubsequently or simultaneously. Further, it has been found in accordancewith the present invention, that chlorous acid, generated by thechemical acidification of chlorite salt or chlorate salt or both canalso be catalyzed in a moist environment to form chlorine dioxide eithersubsequently or simultaneously.

In addition, it has been found in accordance with the present inventionthat additional precursors may be used with the chlorite salt solutionor chlorate salt solution to enhance the catalysis of chlorous acid in amoist environment to form chlorine dioxide either subsequently orsimultaneously. Such precursors include but are not limited topermanganate ion, chloride ion, sodium acid sulfite, peroxide andalcohol.

Still further, it has been found in accordance with the presentinvention that anion exchange materials are a preferred source ofchlorite and/or chlorate ion, exchanged with a counter anion in a moistacidic environment to form chlorous acid, and further catalyzed in themoist environment to form chlorine dioxide either subsequently orsimultaneously. By the ion exchange, a solution of chlorous acid can begenerated from chlorite salt and/or chlorate salt by the saltcation/hydrogen ion exchange. Additionally, ionic contaminants otherwisecontained in the chlorous acid and/or chlorine dioxide solution can beremoved with ion exchange, and ionic stabilizers may be added to thechlorous acid and/or the chlorine dioxide solutions via ion exchange.Still further, the pH of the chlorous acid and/or chlorine dioxidesolutions may be adjusted by the use of ion exchange.

It is, therefore, an object of the present invention to generatechlorine dioxide from chlorous acid in the presence of at least onecatalytic material in either a continuous or batch process in an aqueoussolution or otherwise aqueous moist environment.

Another object of the present invention is to generate a chlorous acidsolution generally free of cations, except hydrogen ion, in either acontinuous or batch process, in an aqueous solution or otherwise aqueousmoist environment.

A further object of the present invention is to generate chlorous acidand chlorine dioxide simultaneously in the presence of at least onecatalytic material in either a continuous or batch process in an aqueoussolution or otherwise aqueous moist environment.

A still further object of the present invention is to utilize an anionexchange material to supply chlorite ion and/or chlorate ion for thegeneration of chlorous acid in either a continuous or batch process inan aqueous solution or otherwise aqueous moist environment.

Still another object of the present invention is to purify the resultingchlorous acid and/or chlorine dioxide solution to remove any ioniccontaminants by the use of ion exchange in either a continuous or batchprocess.

Yet a further object of the present invention is to adjust the pH ofeither the chlorous acid solution and/or the chlorine dioxide solutionby use of ion exchange in either a continuous or batch process.

A final object of the present invention to be stated herein is to addionic stabilizers to either the chlorous acid solution and/or thechlorine dioxide solution by using ion exchange in either a continuousor batch process.

These together with other objects and advantages, which will becomesubsequently apparent, reside in the details of the technology as morefully hereinafter described and claimed, reference being had to theaccompanying drawings forming a part hereof, wherein like numerals referto like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded elevational view of a plastic tube used in thetests of Examples 1-11 described in this application.

FIG. 2 is a graph showing the chlorine dioxide concentration versus timeof a decomposing chlorous acid solution generated by ion exchange.

DETAILED DESCRIPTION OF THE INVENTION

In describing the present invention, specific terminology will be usedfor the sake of clarity. However, the invention is not intended to belimited to the specific terms so selected, or to the specificembodiments disclosed. It is to be understood that each specific termincludes all technical equivalents, which operate in a similar manner toaccomplish a similar purpose, and the specific embodiments are intendedto illustrate, but not limit, the broad technical application andutility of the present invention.

As used herein, the term “solution” shall mean a mixture formed by aprocess by which a solid, liquid, or gaseous substance is mixed with aliquid, whether that liquid is a droplet, aerosol, vapor, or mist. Also,as used herein, the term “moist environment” shall mean that theenvironment in which the reaction occurs contains water moisture,ranging from a slightly humid environment to fully wet. Also, as usedherein, the term “precursor” shall be used to mean any solution and/orcombination of solutions used to generate chlorous acid and/or chlorinedioxide.

It is well known to those knowledgeable of the manufacture of chlorinedioxide that chlorous acid is formed by the acidification of chloritesalt and/or chlorate salt by the reactions (1), (4), and (5). In thesereactions, hydrogen ion is placed in solution where it partiallyacidifies the chlorite salt and/or chlorate salt. The equilibriumconditions of the solution prevent the total acidification of thechlorite and/or chlorate salts, however, because sodium ion remains insolution. It has now been surprisingly found that if the sodium ion isreplaced by a hydrogen ion by means of a cation exchange material, theequilibrium conditions of the solution change, and total acidificationof the chlorite salt and/or chlorate salt is possible, thereby making apure chlorous acid, solution.

Chlorous acid and aqueous solutions containing chlorous acid areparticularly useful in applications where low-level disinfection over along period of time is desirable. Some of these applications includedisinfection of skin, the mouth, and cow teats. In addition, chlorousacid has a relatively low volatility level, making it applicable forsurface disinfection in environments where off-gassing could be harmful.However, due to relatively high levels of residual chlorite in chlorousacid solutions and their inability to rapidly disinfect, chlorinedioxide is preferred in applications such as the disinfection ofdrinking water, cooling towers, food, and surfaces. In addition,chlorine dioxide is desirable for oxidizing organic contaminants andreducing iron and manganese levels in drinking water.

Further, it has been surprisingly discovered that a chlorous acidsolution can be readily catalyzed to form chlorine dioxide. The sourceof the chlorous acid solution can be either that generated by ionexchange or by conventional acidification. If the latter, the catalyticconversion of the chlorous acid to chlorine dioxide drives theacidification reaction to completion or substantial completion.

By definition, catalysts work by changing the activation energy for areaction, i.e. the minimum energy needed for the reaction to occur. Thisis accomplished by providing a′ new mechanism or reaction path throughwhich the reaction can proceed. When the new reaction path has a loweractivation energy, the reaction rate is increased, and the reaction issaid to be catalyzed. When catalysis is used to generate chlorinedioxide from chlorous acid in the present invention, it was surprisinglyfound that neither high concentrations of precursor solutions nor highconcentrations of chlorous acid were required to initiate the reactions.Further, it was surprisingly discovered that the reactions proceededtoward completion rapidly, thus decreasing the opportunity forundesirable byproducts or unreacted precursor materials to contaminatethe chlorine dioxide solutions.

There are many catalysts that can be used within the scope of thepresent invention. These include, but are not limited to platinum,palladium, manganese dioxide, carbon, and ion exchange material.Further, it is well known that depositing such catalysts on varioussubstrates, such as zeolites, aids in the catalysis by increasingsurface area. Such catalysts are commercially available, and it iswithin the scope of those skilled in the art to choose an appropriatecatalytic material and/or substrate to catalyze chlorous acid tochlorine dioxide.

Further, it has been discovered that an anion exchange material can beused to contribute a controlled amount of anions to the precursor,chlorous acid solution, and/or chlorine dioxide solution. For example,the chloride ion must be present for chlorous acid to be generated froma chlorate salt precursor. Although the necessary chloride may bepresent from the decomposition of chlorate, anion resin in the chlorideform may be used to contribute additional chloride ion to the acidifiedchlorate salt precursor.

Ion exchange material can also be used to remove unwanted ions from theprecursor, chlorous acid, and/or chlorine dioxide solution. For example,if the reaction to chlorine dioxide does not go to completion, unreactedchlorite and/or chlorate anion will be present in the chlorine dioxidesolution. Anion exchange material can be used to remove the chloriteand/or chlorate ion. Further, if the precursor solution is acidifiedchemically, excess sodium ion will be present in the chlorine dioxidesolution. Cation exchange material can be used to remove the sodium ion.

Ion exchange materials, such as inorganic and organic resins, membranes,powders, gels, and solutions are well known to those skilled in the art,and the type of ion exchange material used does not limit the invention.Examples of ion exchange materials are weak acid cation resins andpowders, strong acid cation resins and powders, weak base anion resinsand powders, strong base anion resins and powders, sulfonatedpolystyrene solutions, cation and anion selective membranes. Selectionof a particular ion exchange material is considered within the skill ofthose knowledgeable in the field.

In one form of the present invention, cation exchange material is usedto exchange the salt cation in a chlorite precursor with hydrogen ion toform chlorous acid. The resulting chlorous acid is then placed incontact with a catalytic material for a time sufficient to form chlorinedioxide. If the chosen catalyst is able to perform oxidation, such asmanganese dioxide on the surface of greensand, reaction (3)predominates, and 100% of the chlorous acid can convert to chlorinedioxide. However, if the chosen catalyst is unable to perform oxidation,such as platinum, reaction (2) predominates, and only 80% of thechlorous acid can convert to chlorine dioxide.

In another form of the present invention, acid is added to the chloriteprecursor to form chlorous acid with the salt cation still present insolution. The chlorous acid is then placed in contact with a catalyticmaterial for a time sufficient to form chlorine dioxide. The choice ofwhich acid to use depends upon the application. For example, if thechlorous acid and/or chlorine dioxide solution is to be used in a foodprocessing application, an acid such as acetic acid may be preferred. Ifthe chlorous acid and/or chlorine dioxide solution is to be used in ahigh purity industrial application, electrochemically-generated acid maybe used. The choice of acid is well within the scope of knowledge ofthose skilled in this technology.

In another form of the present invention, an acidic reducing agentprecursor is added to the chlorate precursor as the chlorate precursoris placed in contact with a catalytic material for a time sufficient tocause the generation of chlorous acid and chlorine dioxidesimultaneously. If hydrochloric acid is used, it supplies both theacidity and the chloride required for the reaction. However, any acidsource may be used, and the necessary chloride may come from thedecomposition of the chlorate ion.

In another form of the present invention, an acid precursor and areducing agent precursor are added to the chlorate precursor as thechlorate precursor is placed in contact with a catalytic material for atime sufficient to cause the generation of chlorous acid and chlorinedioxide simultaneously. Any acid source may be used, and the necessarychloride may come from the decomposition of the chlorate ion.

In another form of the present invention, a chlorate precursor is placedin contact with a cation exchange material mixed with a catalyticmaterial. The salt cation in the chlorate precursor is exchanged withhydrogen ion as the chlorate precursor contacts both the cation exchangematerial and the catalytic material for a time sufficient to cause thegeneration of chlorous acid and chlorine dioxide simultaneously. Ifnecessary, other precursors, such as sodium chloride, may be dosed alongwith the chlorate precursor to aid in the reaction.

In yet another form of the present invention, a reducing agent is placedin contact with the chlorate precursor either prior to the precursorbeing placed in contact with the catalytic material or as the precursoris placed in contact with the catalytic material for a time sufficientto form chlorine dioxide. In this form of the present invention, thecatalytic material aids in reactions (10), (11), (12), and (13).

In another form of the present invention, a mixed chlorite and chlorateprecursor is acidified as it is placed into contact with a catalyticmaterial for time sufficient to form chlorine dioxide.

DESCRIPTION OF SPECIFIC EMBODIMENTS AND EXAMPLES Precursor Solution forExamples 1-6

In Examples 1-6, a single chlorite precursor solution was used for allExamples. The chlorite precursor solution was prepared by diluting anaqueous 25% sodium chlorite solution with reverse osmosis water. The pHof the resultant solution was measured to be 8.5 with a Hach Sension 1pH meter. The chlorite concentration in the precursor solution wasmeasured to be 823 mg/L by using a Hach Digital Titrator, IodometricTest Kit for Chlorine. To begin the measurement, 100 ml of reverseosmosis water was placed in a 250-ml Erlenmeyer flask, and 2 ml of thesample precursor solution was placed into the reverse osmosis water. OnePotassium Iodide Powder Pillow and one Dissolved Oxygen Reagent 3 Pillowwere added to the solution in the flask, swirled to mix, and placed inthe dark for 10 minutes to allow the reaction to go to completion. Usinga 0.113 N Sodium Thiosulfate Cartridge in the Digital Titrator, thesolution was titrated to a pale yellow. Next, Starch Indicator Solutionwas added until the solution turned blue. The solution was titratedagain until the solution remained colorless for 30 seconds. The titratorreading was recorded and divided by 800 to determine the milliliters oftitrant used. Then the values were plugged into the following formula todetermine the concentration of chlorite in the precursor solution:

$\frac{{ml}\mspace{14mu} {of}\mspace{14mu} {titrant} \times {normality}\mspace{14mu} {of}\mspace{14mu} {sodium}\mspace{14mu} {thiosulfate} \times 67\text{,}450}{{ml}\mspace{14mu} {of}\mspace{14mu} {sample} \times 4}$

Plastic Testing Tubes for Examples 1-10

A sample of the plastic tubes used for carrying out the tests set forthin Examples 1-10 in the present application is shown in FIG. 1 andgenerally designated by reference numeral 100. The plastic test tube 100includes a generally cylindrical body 102 having a conventionalconnection closure mounted at each end in the form of an inlet bottomconnection 104 and an outlet upper end connection 106. Porex supportmedia was cut to fit the inner diameter of the cylindrical tube 102, anda Porex disk 108 was placed at each end between the end of thecylindrical tubing 102 and the end closures 104 and 106 to act assupport for the filling. The feed tubing ran to the inlet bottomconnection 104 and outlet tubing ran from the outlet upper endconnection 106.

Example 1 Chlorous Acid Generation by Cation Exchange

In Example 1, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the tube. The producttubing ran from the top of the tube to a brown sample bottle. In thisexample, the tube was filled with a commercially available strong acidorganic cation resin in the hydrogen form, sold under the name ResintechCG-8, such that the tube was full.

A continuous stream of the chlorite precursor solution was passedupwardly through the tube such that the flow rate was 30 ml/min. A 260ml sample of solution was taken from the tube's top end and placed inthe brown bottle, sealed, and stored in a dark cabinet. A Hach 2010Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L) was used to test the stored sample for chlorinedioxide at one-hour intervals for eight hours.

The results of the foregoing test demonstrate, first, that chlorous acidmay be generated from a chlorite precursor when placed in contact with acation exchange material. Second, the development of chlorous acid tochlorine dioxide over time in this test are shown in FIG. 2, whichdemonstrates the slow reaction time for chlorous acid to decompose tochlorine dioxide.

Example 2 Chlorous Acid Generation by Cation Exchange from a ChloritePrecursor and Subsequent Catalytic Chlorine Dioxide Generation

In Example 2, two identical 30 ml plastic test tubes 100 as shown inFIG. 1 were clipped to a wall with pipe clips. Interconnecting plastictubing ran from the first test tube to the second so that solutionflowed from the bottom to the top of each test tube. The feed tubing ranfrom a reservoir containing the precursor solution to the bottom of thefirst test tube. The product tubing ran from the top of the second testtube to the flow-through cell of a Hach 2010 Spectrophotometer usingMethod 8138 for the measurement of chlorine dioxide (0-700 mg/L).

(A) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin in the hydrogen form such that the tube was full.The second test tube was packed with a commercially available inorganiccation resin in the hydrogen form, sold under the name ResintechSIR-600, having platinum catalyst placed on the surface of the inorganiccation resin such that the tube was full. To place the platinum on thesurface of the Resintech. SIR-600 resin, a 100-ml solution of platinnicchloride was made such that the solution contained 1 gram of platinum.The platinnic chloride solution was then sprayed in a fine mist onto thesurface of one liter of Resintech SIR-600 resin so as to form an evencoating. The coated Resintech SIR-600 resin was then placed in an ovenat 550° F. for three hours. Although platinnic chloride was used in thistest, any soluble platinum salt could have been used for the coatingmaterial. Such salts and methods are well known to those schooled in theart of catalysis, and many are readily available as standard products, Acontinuous stream of the chlorite precursor solution was passedsequentially through the first and then the second test tube such thatthe flow rate was 30 ml/min.

(B) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin in the hydrogen form such that the tube was full.The second, test tube was packed with acid-washed carbon particles suchthat the drying tube was full. A continuous stream of the chloriteprecursor solution was passed sequentially through the first and thenthe second test tube such that the flow rate was 30 ml/min.

(C) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin in the hydrogen form such that the tube was full.The second test tube was packed with acid-washed carbon particles havingplatinum catalyst placed on the surface of the acid-washed carbon suchthat the tube was full. The platinum was placed on the surface of theacid-washed carbon The platinum was placed on the surface of theacid-washed carbon particles by the same method described in Example2(A) above for depositing platinum on the surface of the cation resin. Acontinuous stream of the chlorite precursor solution was passedsequentially through the first and then the second test tube such thatthe flow rate was 30 ml/min.

(D) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin in the hydrogen form such that the tube was full.The second test tube was packed with regenerated manganese greensandsuch that the tube was full. A continuous stream of the chloriteprecursor solution was passed sequentially through the first and thenthe second test tube such that the flow rate was 30 ml/min.

(E) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin in the hydrogen form such that the tube was full.The second test tube was packed with the Resintech SIR-600 inorganiccation exchange resin having manganese dioxide placed on the surface ofthe inorganic cation exchange material such that the tube was full. Toplace the manganese dioxide on the surface of the Resintech SIR-600resin, a 100-ml solution of manganese sulfate was made such that thesolution contained 1 gram of manganese. The manganese sulfate solutionwas then sprayed in a fine mist onto the surface of one liter ofResintech SIR-600 resin so as to form an even coating. The coatedResintech SIR-600 resin was then placed in an oven at 550° F. for threehours which converted the manganese to manganese dioxide. Althoughmanganese sulfate was used in this test, any soluble manganese saltcould have been used for the coating material. Such salts and methodsare well known to those schooled in the art of catalysis, and many arereadily available as standard products. A continuous stream of thechlorite precursor solution was passed sequentially through the firstand then the second test tube such that the flow rate was 30 ml/min.

(F) The first test tube was filled with the Resintech CG-8 strong acidorganic cation resin, in the hydrogen form such that the tube was full.The second test tube was packed with a chlorite regenerated form of acommercially available organic anion exchange material having palladiumon the resin, sold under the name Lewatit K7333 by Bayer. Corporation,such that the tube was full. As purchased, the resin is in the hydroxylform, but for this example, the resin was regenerated with sodiumchlorite solution so as to place the resin in the chlorite form. Acontinuous stream of the chlorite precursor solution was passedsequentially through the first and then the second test tube such thatthe flow rate was 30 ml/min.

The results of the tests in Examples 2(A) through 2(F) are shown in thefollowing Table 1.

TABLE 1 Chlorine Dioxide Example No. Concentration (mg/L) 2(A) 575 2(B)427 2(C) 526 2(D) 549 2(E) 804 2(F) 284

Table 1 shows the concentration of chlorine dioxide after the chlorousacid from the first tube has been catalytically converted to chlorinedioxide in the second tube in the tests of Examples (A) through (F).

It will be seen that chlorous acid is significantly more readilyconverted to chlorine dioxide in the presence of a catalyst, as shown inall of Examples 2(A) through 2(F), compared to known chemicaldecomposition, as shown in Example 1, in which the chlorine dioxideconcentration reaches only about 130 mg/L after eight hours, see FIG. 2

Example 3 Simultaneous Generation of Chlorous Acid and Chlorine Dioxideby a Combination of Cation Exchange and Catalysis from a ChloritePrecursor

In Example 3, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the tube. The producttubing ran from the top of the tube to the flow-through cell of a Hach2010 Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L). In this example, the test tube 100 was packed witha 50/50 mixture of the Resintech CG-8 strong acid organic cation resinin the hydrogen form and the Resintech SIR-600 inorganic cation resin inthe hydrogen form having platinum catalyst placed on the surface of theinorganic cation resin in the method as described above in Example 2 (A)such that the test tube was full.

A continuous stream of the chlorite precursor solution was passedthrough the test tube such that the flow rate was 30 ml/min. Theresultant concentration of chlorine dioxide from the test tube was 522mg/L. It will thus be seen that the simultaneous generation of chlorousacid and chlorine dioxide readily occurs from a chlorite precursorsolution in the presence of a cation exchange material and suitablecatalyst.

Example 4 Simultaneous Generation of Chlorous Acid and Chlorine Dioxideby a Combination of Acidification and Catalysis from a ChloritePrecursor

In Example 4, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the test tube. Theproduct tubing ran from the top of the test tube to the flow-throughcell of a each 2010 Spectrophotometer using Method 8138 for themeasurement of chlorine dioxide (0-700 mg/L). In this example, the testtube 100 was packed with the Resintech SIR-600 inorganic cation resin inthe hydrogen form having platinum catalyst placed on the surface of theinorganic cation resin in the method as described above in Example 2(A)such that the test tube was full.

A continuous stream of the chlorite precursor solution was acidified toa pH of 2.5 and passed through the test tube such that the flow rate was30 ml/min. The resultant concentration of chlorine dioxide from the testtube was 522 mg/L. It will thus be seen that the simultaneous generationof chlorous acid and chlorine dioxide readily occurs from an acidifiedchlorite precursor solution in the presence of a suitable catalyst.

Example 5 Purification of a Chlorine Dioxide Solution with Ion Exchange

(A) In Example 5 (A), two 30 ml plastic test tubes 100 as shown in FIG.1 were clipped to a wall with pipe clips. Interconnecting plastic tubingran from the first test tube to the second so that solution flowed fromthe bottom to the top of each tube. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the first test tube.The product tubing ran from the top of the second test tube to theflow-through cell of a Hach 2010 Spectrophotometer using Method 8138 forthe measurement of chlorine dioxide (0-700 mg/L). The first test tubewas filled with the Resintech CG-8 strong acid organic cation resin inthe hydrogen form such that the tube was full. The second test tube waspacked with the Resintech SIR-600 inorganic cation resin in the hydrogenform having platinum catalyst placed on the surface of the inorganiccation resin in the method as described above in Example 2(A) such thatthe tube was full. A continuous stream of the chlorite precursorsolution was passed sequentially through the test tubes such that theflow rate was 30 ml/min.

(B) In Example 5 (B), three 30 ml plastic test tubes 100 as shown inFIG. 1 were clipped to a wall with pipe clips. Interconnecting plastictubing ran from the first test tube to the second and from the second tothe third so that solution flowed from the bottom to the top of eachtest tube. The feed tubing ran from a reservoir containing the precursorsolution to the bottom of the first test tube. The product tubing ranfrom the top of the third test tube to the flow-through cell of a Hach2010 Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L). The first and second test tubes were filled as inExample 5(A). The third test tube was packed with a 50/50 mixture of theResintech CG-8 strong acid organic cation resin in the hydrogen form anda commercially available weak base organic anion resin in the hydroxylform, sold under the name Resintech WBMP, such that the tube was full. Acontinuous stream of the chlorite precursor solution was passedsequentially through the test tubes such that the flow rate was 30ml/min.

The results of the tests in Examples 5 (A) and 5 (B) are shown in thefollowing Table 2.

TABLE 2 Chlorine Dioxide Example No. Concentration (mg/L) 5(A) 546 5(B)542

Table 2 shows the concentration of chlorine dioxide before purificationby ion exchange (Example 5(A)), and after purification by ion exchange(Example 5(B)). It will thus be seen that the concentration of chlorinedioxide is not affected if the chlorine dioxide solution is purified byion exchange.

Example 6 pH Adjustment of a Chlorine Dioxide Solution with Ion Exchange

(A) In Example 6(A), two 30 ml plastic test tubes 100 as shown in FIG. 1were clipped to a wall with pipe clips. Interconnecting plastic tubingran from the first test tube to the second so that solution flowed fromthe bottom to the top of each tube. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the first test tube.The product tubing ran from the top of the second test tube to theflow-through cell of a Hach 2010 Spectrophotometer using Method 8138 forthe measurement of chlorine dioxide (0-700 mg/L). The first test tubewas filled with the Resintech CG-8 strong acid organic cation resin inthe hydrogen form such that the tube was full. The second test tube waspacked with the Resintech SIR-600 inorganic cation resin in the hydrogenform having platinum catalyst placed on the surface of the inorganiccation resin in the method as described above in Example 2(A) such thatthe tube was full. A continuous stream of the chlorite precursorsolution was passed sequentially through the test tubes such that theflow rate was 30 ml/min. The pH of the resultant solution was 2.4.

(B) In Example 6(B), three 30 ml plastic test tubes 100 as shown in FIG.1 were clipped to a wall with pipe clips. Interconnecting plastic tubingran from the first test tube to the second and from the second to thethird so that solution flowed from the bottom to the top of each tube.The feed tubing ran from a reservoir containing the precursor solutionto the bottom of the first test tube. The product tubing ran from thetop of the third test tube to the flow-through cell of a Hach 2010Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L). The first and second test tubes were filled as inExample 6 (A). The third test tube was packed with an inorganic anionresin, hydrotalcite, in the carbonate form such that the drying tube wasfull. A continuous stream of the chlorite precursor solution was passedsequentially through the test tubes such that the flow rate was 30ml/min. The pH of the resultant solution was 8.1.

The results of the tests in Examples 6(A) and 6(B) are shown in thefollowing Table 3.

TABLE 3 Chlorine Dioxide Example No. Concentration (mg/L) 6(A) 546 6(B)541

Table 3 shows the concentration of chlorine dioxide before and after pHadjustment. It will accordingly be seen that the concentration ofchlorine dioxide is not affected if the pH of the chlorine dioxidesolution is adjusted by ion exchange.

Example 7 Addition of Stabilizing Ion to Chlorine Dioxide Solution

(A) In Example 7 (A), two 30 ml plastic test tubes 100 as shown in FIG.1 were clipped to a wall with pipe clips. Interconnecting plastic tubingran from the first test tube to the second so that solution flowed fromthe bottom to the top of each tube. The feed tubing ran from a reservoircontaining the precursor solution to the bottom of the first test tube.The product tubing ran from the top of the second test tube to theflow-through cell of a Hach 2010 Spectrophotometer using Method 8138 forthe measurement of chlorine dioxide (0-700 mg/L). The first test tubewas filled with the Resintech CG-8 strong acid organic cation resin inthe hydrogen form such that the tube was full. The second test tube waspacked with the Resintech SIR-600 inorganic cation resin in the hydrogenform having platinum catalyst placed on the surface of the inorganiccation resin in the method described above in Example 2(A) such that thetube was full. A continuous stream of the chlorite precursor solutionwas passed sequentially through the test tubes such that the flow ratewas 30 ml/min. The pH of the resultant solution was 2.4.

(B) In Example 7 (B), three 30 ml plastic test tubes 100 as shown inFIG. 1 were clipped to a wall with pipe clips. Interconnecting plastictubing ran from the first test tube to the second and from the second tothe third so that solution flowed from the bottom to the top of eachtube. The feed tubing ran from a reservoir containing the precursorsolution to the bottom of the first test tube. The product tubing ranfrom the top of the third test tube to the flow-through cell of a Hach2010 Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L). The first and second test tubes were filled as inExample 7(A). The third test tube was packed with an inorganic anionresin, hydrotalcite, in the phosphate form such that the tube was full.A continuous stream of the chlorite precursor solution was passedsequentially through the tubes such that the flow rate was 30 ml/min.The pH of the resultant solution was 7.8.

The results of the tests in Examples 7(A) and 7(B) are shown in thefollowing Table 4.

TABLE 4 Chlorine Dioxide Example No. Concentration (mg/L) 7(A) 546 7(B)544

Table 4 shows the concentration of chlorine dioxide before and after theaddition of a stabilizing ion. It will thus be seen that theconcentration of chlorine dioxide is not affected when a stabilizingion, such as phosphate, is added to the chlorine dioxide solution.

Example 8 Simultaneous Generation of Chlorous Acid and Chlorine Dioxideby a Combination of Cation Exchange and Catalysis from a ChloratePrecursor

In Example 8, a precursor solution was made containing both chlorate ionand chloride ion such that the concentration of the solution was 7,830mg/L as chlorate and 4087 mg/L as chloride. The precursor solution wasthen placed in a reservoir and flowed through the apparatus.

One 30 ml plastic test tube 100 as shown in FIG. 1 was clipped to a wallwith pipe clips. The feed tubing ran from a reservoir containing theprecursor solution to the bottom of the test tube. The product tubingran from the top of the tube to the flow-through cell of a Hach 2010Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L). In this example, the test tube was packed the sameas in Example 3.

A continuous stream of the chlorate precursor solution was passedthrough the test tube such that the flow rate was 30 ml/min. Theresultant concentration of chlorine dioxide from the test tube was 93mg/L. It will thus be seen that a simultaneous generation of chlorousacid and chlorine dioxide from a chlorate precursor solution occurs inthe presence of a cation exchange material and suitable catalyst.

Example 9 Simultaneous Chlorous Acid and Chlorine Dioxide GenerationUsing Ion Exchange and a Hydrochloric Acid Precursor

In Example 9, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from agas-washing bottle containing 150 ml of a 30% hydrochloric acidprecursor solution to the bottom of the test tube. The product tubingran from the top of the test tube to a second gas-washing bottle thatcontained 200 ml of reverse osmosis water. The test tube was packed witha chlorate regenerated form of a commercially available organic anionexchange material having palladium on the resin, sold under the nameLewatit K7333 by Bayer Corporation, such that the tube was full. Aspurchased, the resin is in the hydroxyl form, but for this example, theresin was regenerated with sodium chlorate solution so as to place theresin in the chlorate form. The hydrochloric acid vapor was strippedfrom the hydrochloric acid precursor solution with compressed air andthe vapor then flowed through the test tube. The final product from thetest tube was sparged into the water in the second gas-washing bottle.The resultant solution from the second gas-washing bottle was tested forchlorine dioxide with a Hach 2010 Spectrophotometer using Method 8138for the measurement of chlorine dioxide (0-700 mg/L).

A continuous stream of hydrochloric acid precursor vapor was passedthrough the test tube for 5 minutes. After 5 minutes, the concentrationof chlorine dioxide was measured in the second gas-washing bottle. Theresultant concentration of chlorine dioxide from the tube collected inthe second gas-washing bottle was 187 mg/L. It will hence be seen that asimultaneous generation of chlorous acid and chlorine dioxide occurs inthe presence of an acidic reducing agent, a suitable catalyst, and ananion exchange material in the chlorate form.

Example 10 Simultaneous Chlorous Acid and Chlorine Dioxide GenerationUsing Ion Exchange and a Sodium Acid Sulfite Precursor

In Example 10, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from agas-washing bottle containing 150 ml of a 20% sodium acid sulfite(NaHSO₃) precursor solution to the bottom of the test tube. The producttubing ran from the top of the test tube to a second gas-washing bottlethat contained 200 ml of reverse osmosis water. The test tube was packedwith the same organic anion exchange material in the chlorate formhaving palladium on the resin as described in Example 9 such that thetube was full. The sodium acid sulfite vapor was stripped from thesodium acid sulfite precursor solution with compressed air and thesodium acid sulfite vapor then flowed through the tube. The finalproduct from the test tube was sparged into the water in the secondgas-washing bottle. The resultant solution from the second gas-washingbottle was tested for chlorine dioxide with a Hach 2010Spectrophotometer using Method 8138 for the measurement of chlorinedioxide (0-700 mg/L).

A continuous stream of sodium acid sulfite precursor vapor was passedthrough the tube for 5 minutes. After 5 minutes, the concentration ofchlorine dioxide was measured in the second gas-washing bottle. Theresultant concentration of chlorine dioxide from the test tube collectedin the second gas-washing bottle was 576 mg/L. It will therefore be seenthat a simultaneous generation of chlorous acid and chlorine dioxideoccurs in the presence of an acidic reducing agent, a suitable catalyst,and an anion exchange material in the chlorate form.

Example 11 Simultaneous Chlorous Acid and Chlorine Dioxide GenerationUsing an Ion Exchange Catalyst and a Sodium Acid Sulfite Precursor

In Example 11, one 30 ml plastic test tube 100 as shown in FIG. 1 wasclipped to a wall with pipe clips. The feed tubing ran from agas-washing bottle containing 150 ml of a 20% sodium acid sulfite(NaHSO₃) precursor solution to the bottom of the test tube. The producttubing ran from the top of the test tube to a second gas-washing bottlethat contained 200 ml of reverse osmosis water. The test tube was packedwith a known inorganic anion resin, hydrotalcite, regenerated to be inthe chlorate form, such that the drying tube was full. The sodium acidsulfite vapor was stripped from the sodium acid sulfite precursorsolution with compressed air and the sodium acid sulfite vapor thenflowed through the tube. The final product from the test tube wassparged into the water in the second gas-washing bottle. The resultantsolution from the second gas-washing bottle was tested for chlorinedioxide with a Hach 2010 Spectrophotometer using Method 8138 for themeasurement of chlorine dioxide (0-700 mg/L).

A continuous stream of sodium acid sulfite precursor vapor was passedthrough the test tube for 5 minutes. After 5 minutes, the concentrationof chlorine dioxide was measured in the second gas-washing bottle. Theresultant concentration of chlorine dioxide from the test tube collectedin the second gas-washing bottle was 318 mg/L. It will thus be seen thata simultaneous generation of chlorous acid and chlorine dioxide occursin the presence of an acidic reducing agent and a catalytic anionexchange material in the chlorate form.

The foregoing descriptions and examples should be considered asillustrative only of the principles of the invention. Since numerousapplications of the present invention will readily occur to thoseskilled in the art, it is not desired to limit the invention to thespecific examples disclosed or the exact construction and operationshown and described. Rather, all suitable modifications and equivalentsmay be resorted to, falling within the scope of the invention.

1-41. (canceled) 42: A process for producing chlorine dioxide comprisingthe step of: feeding chlorous acid into contact with a catalyticmaterial deposited on a substrate in a moist environment to produce thechlorine dioxide. 43: The process of claim 42, wherein the catalyticmaterial is selected from the group consisting of platinum, palladium,manganese dioxide, carbon, ion exchange material, and combinationsthereof. 44: The process of claim 42, wherein the catalytic material isplatinum. 45: The process of claim 42, wherein the catalytic material ispalladium. 46: The process of claim 42, wherein the catalytic materialis manganese dioxide. 47: The process of claim 42, wherein the catalyticmaterial is carbon. 48: The process of claim 42, wherein the catalyticmaterial is ion exchange material. 49: The process of claim 42, whereinthe catalytic material is platinum coated acid-washed carbon particles.50: A process for generating chlorous acid comprising contacting asolution containing counter anions with an anion exchange material inchlorite form in a moist acidic environment, such that the counteranions of the solution exchange with the chlorite on the anion exchangematerial to form the chlorous acid.